Caller: Not eon

Received a letter supposedly from eon with this number on it claiming that I owe £1,716.78, saying I need to call it or I will be charged £20, threatening also with debt collection agencies and legal action. eon are not my utility supplier. This number is not listed on eon's website (see here https://www.eonenergy.com/contact/phone-numbers.html)

Compared the letter to a genuine one from after I moved in (previously supplied the property), the paper is different, the font is different, not to mention that it is addressed to "The Occupier" and opens with "Dear Customer" - doesn't have my name anywhere on it. The account number is formatted differently (genuine letter had 3 groups of 4 digits, this one had 1 set of 12).

The best part is the address it had in the bottom-right corner, "Residential Collections, PO Box 10360" with the postcode NG1 9JG, which, according to the Royal Mail and Post Office postcode finders (and Google Maps), does not exist.